The present invention relates to a butadiene polymer composition superior in resistance to thermal degradation and discoloration.
Butadiene polymers such as solution-polymerized polybutadiene rubber (BR), solution-polymerized styrene-butadiene copolymer rubber (SBR) and styrene-butadiene block copolymer (SBS) are generally produced by anionic polymerization in a hydrocarbon solvent using a Ziegler catalyst or an organolithium compound as a catalyst. Removal of the solvent from a polymer solution after completion of the polymerization has hitherto been performed by a steam-stripping method, but recently, a method of directly removing the solvent which can theoretically minimize the amount of steam required has been proposed for saving of energy.
However, the latter process is normally carried out at a high temperature of about 150.degree.-200.degree. C. which is considerably higher than the boiling point of polymerization solvents. Therefore, this method suffers from the problems such as gel formation and discoloration of polymers after the high temperature process. Such being the case, there has been desired to improve resistance to thermal degradation and discoloration in the preparation of butadiene polymers, especially in the absence of oxygen.
Furthermore, in extrusion molding or injection molding of SBS or high-impact polystyrenes modified with BR, SBR or SBS, high temperature and high speed processing is required and owing to insufficient thermal resistance, fish eye gel often occurs to cause the problems such as deterioration of film properties and discoloration of the film. Thus, solution of these problems has been earnestly demanded.
It has been well known to use various antioxidants of phenol type, phosphorus type and sulfur type during preparation and processing of butadiene polymers. For example, phenolic antioxidants such as 2,6-di-t-butyl-4-methylphenol, 2,2'-methylenebis(6-t-butyl-4-methylphenol), n-octadecyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, triethylene glycol bis[3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionate], pentaerythrityl tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]and 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene are used alone or in combination with phosphorus type antioxidants such as tris(nonylphenyl) phosphite and distearyl pentaerythrityl diphosphite or in combination with sulfur type antioxidants such as dilauryl thiodipropionate, dimyristyl thiodipropionate, distearyl thiodipropionate and pentaerythrityl tetrakis(3-laurylthiopropionate).
However, these methods are not sufficient to prevent thermal deterioration (gelation) which may occur, especially in the absence of oxygen, at a high temperature process for separation of the polymer from a polymer solution in the preparation of butadiene polymers or at high temperature processing of butadiene polymers.
Further, a phenolic compound represented by the following formula (Ia) has been known as a stabilizer for butadiene polymers. ##STR3##
For example, U.S. Pat. No. 4,525,514 discloses a compound of the above formula (Ia) where R.sub.1 is hydrogen, R.sub.2 is an alkyl of 1-4 carbon atoms and R.sub.3 is t-butyl. It mentions that particularly, the compound where R.sub.2 is methyl, namely, 2-t-butyl-6-(3-t-butyl-2-hydroxy-5-methylbenzyl)-4-methylphenyl acrylate is effective as a stabilizer for butadiene polymers. This stabilizer exhibits the effect to prevent thermal deterioration (gelation), especially in the absence of oxygen, at a high temperature process for separation of the polymer from a polymer solution in the preparation of butadiene polymers or at high temperature processing of butadiene polymers, but it has become apparent that the use of this stabilizer has the problem in that the separated polymer is practically insufficient in its hue.
Moreover, as an antioxidant for various synthetic resins for preventing discoloration caused by oxidative deterioration in the presence of oxygen, U.S. Pat. No. 4,365,032 has proposed a monoester compound of 2,2'-alkylidenebis(4,6-di-alkyl substituted phenol) including the acrylate represented by the above mentioned formula (Ia). This patent specifies R.sub.1 in the formula (Ia) to be hydrogen or an alkyl of 1-10 carbon atoms and R.sub.2 and R.sub.3 to be an alkyl of 1-4 carbon atoms, a cycloalkyl of 5-6 carbon atoms or a methyl-substituted cycloalkyl of 5-6 carbon atoms. However, this patent makes no mention of preventing discoloration or thermal deterioration (gelation) which may occur, especially in the absence of oxygen, at a high temperature process for separating a polymer from a polymer solution in the production of butadiene polymers or at high temperature processing of butadiene polymers. Besides, the compounds specifically exemplified in the patent showed no sufficient effect to prevent thermal deterioration or discoloration, especially in the absence of oxygen, at a high temperature process in the production of butadiene polymers or at high temperature processing of butadiene polymers.
Furthermore, Japanese Patent Kokai No. 62-223248 has proposed combination use of the phenolic compound represented by the above formula (Ia) with other phenolic compound and/or a phosphorus compound to improve processing stability and discoloration of polyethylene. This patent publication merely mentions stabilization of polyethylene and makes no teaching to stabilize butadiene polymers, especially to prevent the thermal deterioration (gelation) or discoloration in the absence of oxygen.
The patent publication generally defines R.sub.1 in the formula (Ia) to be hydrogen or methyl and R.sub.2 and R.sub.3 to be hydrogen or an alkyl of 1-8 carbon atoms. In its Examples, 2-t-butyl-6-(3-t-butyl-2-hydroxy-5-methylbenzyl)-4-methylphenyl acrylate (a compound of the formula (Ia) where R.sub.1 is hydrogen, R.sub.2 is methyl and R.sub.3 is t-butyl) is used in combination with other phenolic compound and/or a phosphorus compound.
According to the inventors' investigation, when the phenolic compound and the phosphorus compound described in the above patent publication are used in combination for butadiene polymers, effect can be exhibited to some extent against thermal deterioration (gelation) or discoloration caused, especially in the absence of oxygen, at a high temperature process for separating a polymer from a polymer solution in the production of butadiene polymers or at high temperature processing of butadiene polymers, but sufficient effect cannot be exhibited against discoloration due to oxides of nitrogen gas in practical use.